Direct cellular cytotoxicity by sheep peripheral blood leukocytes (PBL) has previously been shown to occur when these cells were separated by ficoll-lypaque and added to sheep skin fibroblasts infected with vaccinia virus. Greater levels of cytotoxic activity (Cr-51 release) were consistently achieved with autologous infected target cells than with allogeneic or xenogeneic targets. Proposed work will be designed to detect genetic destriction of this type of anti-viral immunity. Attempts will be made to identify and characterize the effector cell populations. Curiously, efferent lymphocytes (EL) from a popliteal lymph node responding to a pox virus infection failed to react under the same conditions in which PBL are active. Experiments are to be performed to help determine why EL are inactive or if they will become active when cultural conditions are modified.